This invention relates to the separation of platinum group metals from various feedstock materials in a form suitable for further separation and purification.
Prior art pyrometallurgical methods for recovery of platinum group metals, sometimes referred to herein as "PGM's", from various feedstock materials by concentrating them in collector metals have not given entirely satisfactory results--in part--due to the long periods of time (residence time) required for the PGM's to accumulate in the collector metal and separate into a recoverable layer. This necessitates providing a multiplicity of sizes and types of furnaces for treatment of various feedstock materials.
For example, in processes employing electric arc furnaces the slag is heated by passing an electric current between submerged electrodes, through molten slag causing localized heating and temperature gradients which result in significant viscosity gradients in the melt. Higher slag viscosity impedes aggregation and settling of very fine particles of PGM's and collector metals as well as movement of the slag and thus slows the formation of a recoverable layer of PGM's associated with collector metal.
Another disadvantage of prior art processes for recovery of PGM's from finely divided material is a frequent requirement for pre-processing of the feedstock materials into forms that facilitate separation of the PGM's e.g. pelletization. As is well known in the art, pelletization involves comminution and mixing the feedstock material with appropriate fluxes, collector metals, binder and the like, and processing the mixture into larger particles of sufficient size and mass so that they form an open-structured layer on the slag surface and are carried, relatively intact, to the heating zone of whatever furnace is being used. Thus problems associated with segregation of the melt constituents and escape of reaction gases are avoided.
Another disadvantage of prior art proceseses is low tolerance for treating different types of feedstock material.
An exemplary feedstock material is PGM concentrates produced from chromite-bearing ore by processes including comminution, magnetic separation mineral dressing, flotation, and the like. The PGM's which include platinum, palladium, rhodium, ruthenium, iridium and osmium, are sometimes found in association with chromite-bearing ores at chromite grain boundaries, within chromite grains or in the gangue material associated with the ore and they are usually also associated with sulphides of nickel, copper and iron. Extensive deposits of platinum group metals associated with chromite bearing ores exist in the Republic of South Africa and the U.S.A., in particular, the Stillwater Complex in Montana. Of course, the many industrial forms of PGM's results in a large number of additional feedstock materials, other than ores, in which they may be found. Therefore, a versatile process that can recover PGM's from a variety of different feedstock materials, economically and efficiently, is very desirable. Typically, chromite occurs as stratiform or podiform deposits associated with ultramafic igneous rocks. PGM's are of significant industrial value finding application, for example, as catalytic or inert materials in many chemical reactions. They are used extensively in the petroleum industry as catalysts, in the making of dies for the manufacture of fiberglass, in the electrical industry for switch contacts, and for treating automotive exhaust gases in catalytic converters to render harmless oxides of nitrogen, carbon and sulphur. Other uses are for dental devices and jewelry. The major commercial production of platinum group metals from ores is limited to the Republic of South Africa, U.S.S.R., and Canada although there are recycling, purifying and fabricating facilities in many countries.
A traditional method for extracting platinum group metals from ores containing little or no chromite, such as the Merensky Reef ore in the Republic of South Africa, consists of comminution and flotation to produce a concentrate containing platinum group metals and sulphides of nickel, copper and iron. The concentrate is smelted in a continuous process with an average residence time of several hours in a submerged arc, carbon electrode furnace to form a metal matte, to which the platinum group metals report, and slag. The iron and sulphur in the matte are subsequently removed in a separate process step consisting of an air blast converter to which silica is added for reaction with the iron to form a fayalite slag. The slag is recycled in liquid form to the electric arc furnace for reheating and recovery of any entrained particles containing platinum group metals and ultimate discharge from the electric arc furnace as waste. The product from the converter is granulated and treated electrolytically to separate the nickel and copper and to produce a residue containing PGM's in a form suitable for separation and purification of the individual platinum group metals.
It has been found that if chromite-bearing ore containing platinum group metals is treated by this method, the residual chromite particles in the PGM feedstock interfere with the process steps and cause losses of platinum group metals and undesirable accretions in the furnace. It appears that chromite reacts with the carbon electrode material in electric arc furnaces to form ferrochrome which alloys with the platinum group metals and from which the platinum group metals cannot be readily extracted. In addition, chromite particles remote from the electrodes appear to settle out on the furnace walls and hearth forming the above-mentioned undesirable accretions which interfere with smooth operation of the furnace.